It is conventionally known that when performing plastic forming, a dimensional accuracy of the work lowers due to the elastic recovery (spring back) that appears when restriction by a molding tool, such as the die which applies deformation on the material to be processed, is released from the work.
Numerous automobile parts have a complex shape such as curving shape or cross-sectional shape changing at any portion, and thus identification of the cause of elastic recovery of deformation is difficult and it takes time to investigate the cause. The cause of elastic recovery of deformation differs depending on the shape and the material of the work, and thus it was difficult to perform appropriate countermeasures on the elastic recovery of deformation for every work in a short period of time after clarifying the cause of elastic recovery of deformation.
Thus, instead of excluding the cause of elastic recovery of deformation that lowers the dimensional accuracy, in order to reduce the shape defect of the work arising from the elastic recovery of the deformation, the shape of the molding tool such as the die is designed so that design dimension is obtained in a state the deformation applied to the work is elastically recovered.
For instance, JP-2003-33828-A proposes a simulation technique of calculating the stress distribution in a state where the plate material is pressed to a molding bottom dead center of the die using elastoplastic finite element analysis, calculating the elastic recovery amount of the plate material based on the stress distribution, and correcting the die so as to match the shape of the plate material after elastic recovery in the press molding process of molding a plate material (material to be processed) with a die (molding tool).
In recent years, the high strength steel plate is widely used in automobile constituting parts and has a large elastic recovery amount of deformation by plastic forming, and thus demand for a more advanced solution is increasing on the reduction of dimensional accuracy failure. In addition, the developing period of the automobile has been shortened recently, and development of the plastic forming technique capable of rapidly and reliably reducing the dimensional accuracy failure is being desired.
As described above, the simulation technique of determining the shape of the molding tool so that the shape of the work after elastic recovery matches the designed shape is also effective on the high strength steel plate, but the elastic recovery amount of deformation during the molding process is larger in the high strength steel plate in comparison with the normal steel plate, and it is sometimes difficult to create the molding tool in view of the elastic recovery of deformation. Therefore, it is required to develop a technique of clarifying the cause for elastic recovery of deformation in the molding process, capable of removing the cause, resolving lowering in dimensional accuracy, and of preventing defect in shape.